Prion diseases are disease of protein conformation. Determination of the conformation differences between different forms of the prion protein (PrP) is central to understanding the nature of these diseases. However the physical forms of PrP, e.g. as a membrane-associated protein or in insoluble aggregates, complicate this determination. Our approach is to use monoclonal antibodies as sensitive specific probes of protein conformation which can be employed in a wide range of environments. The use of these probes, in conjunction with the detailed structural data available from NMR and crystallography of a limited number of PrP molecules and fragments, allows the conformations of PrP in many forms and milieu to be explored. Earlier problems in generating antibodies to PrP have been circumvented by immunizing PrP-gene ablated mice and rescuing antibodies via phage display libraries. This approach has yielded a wealth or recombinant antibodies that clearly define differences in epitope exposure between the normal cell surface PrP molecule and the protease resistant core of infectious prion particles. The generation of new forms of infective PrP molecules described in this program provides new opportunities, using the antibodies and approaches developed, to elucidate the critical changes involved in the acquisition of prion infectivity. Particular attention is to be focussed on a PrP molecule with a triple A-V mutation and a shortened PrP molecule (PrP106). Both of these molecules appear to adopt conformation which are scrapie- like in that they are associated with infectivity or protease resistance. We propose to investigate these molecules in vitro and in situ using the panel of existing antibodies and to use the molecules as immunogens to generate new antibodies. The conformations of new PrP molecules generated as Project 1 develops will similarly be probed by the antibody approach.